Dipole moments covalent bonds

Bond Dipole Moments (Debye) for Some Common Covalent Bonds... 

In a very large number of compounds the chemical bond cannot be considered as purely ionic or purely covalent the dipole moments of such bonds are greater than zero but considerably less than the values given in Table XXV. Typical examples of such bonds are HF, HCl, HBr, HI, NH, OH, GCl etc. If the experimental values for the bond energies of the hydrogen halides are compared with those calculated by the Pauling s method it is found Table XXVI) that for HI the calculated value is greater than the experimental. Such a result is clearly impossible since, because of resonance which was not taken into account... 

In Section 1.3, we saw that for molecules with one covalent bond, the dipole moment of the bond is identical to the dipole moment of the molecule. For molecules that have more than one covalent bond, the geometry of the molecule must be taken into account because both the magnitude and the direction of the individual bond dipole moments (the vector sum) determine the overall dipole moment of the molecule. Symmetrical molecules, therefore, have no dipole moment. For example, let s look at the dipole moment of carbon dioxide (CO2). Because the carbon atom is bonded to two atoms, it uses sp orbitals to form the C—O a bonds. The remaining two p orbitals on carbon form the two C—O tt bonds. The individual carbon-oxygen bond dipole moments cancel each other— because sp orbitals form a bond angle of 180°—giving carbon dioxide a dipole moment of zero D. Another symmetrical molecule is carbon tetrachloride (CCI4). The four atoms bonded to the sp hybridized carbon atom are identical and project symmetrically out from the carbon atom. Thus, as with CO2, the symmetry of the molecule causes the bond dipole moments to cancel. Methane also has no dipole moment. 

The polarity of a covalent bond is measured by a vector quantity called a bond dipole moment and is given the symbol /t (Greek mu). Bond dipole moment is defined as the product of the charge, e (either the 5+ or 8 because each is the... 

I Table 1.7 Average Bond Dipole Moments of Selected Covalent Bonds... 

Bond dipole moment (fi) (Section 1.2B) A measure of the polarity of a covalent bond. The product of the charge on either atom of a polar bond times the distance between the atoms. 

When two atoms of differing electronegativites are bonded, one end of the bond will be 5+ and the other will be 5—. This analysis leads to the notion of a bond dipole as the local moment that is associated with a polar covalent bond. A moment reflects the electrostatic force that would be exerted by a charge on a neighboring charge. The dipole moment pro-... 

In Section 1.3, we saw that if a molecule has one covalent bond, then the dipole moment of the molecule is identical to the dipole moment of the bond. When molecules have more than one covalent bond, the geometry of the molecule must be taken into account because both the magnitude and the direction of the individual bond dipole moments (the vector sum) determine the overall dipole moment of the molecule. 

In addition to the in situ electronegativity of atom A, EIa, there is also the question of in situ bond polarity across the bond AB in a molecule. At first, one would expect this to be IEa — IEb, but bond polarity is the ionic component (bond dipole moment) of the AB bond, and thus the covalent component must be subtracted out. So the bond polarity index, BPIab, must be defined as... 

The unequal distribution of charge produced when elements of different electronegativities combine causes a polarity of the covalent bond joining them and, unless this polarity is balanced by an equal and opposite polarity, the molecule will be a dipole and have a dipole moment (for example, a hydrogen halide). Carbon tetrachloride is one of a relatively few examples in which a strong polarity does not result in a molecular dipole. It has a tetrahedral configuration... 

Carbon-oxygen and carbon-halogen bonds are polar covalent bonds and carbon bears a partial positive charge in alcohols ( " C—0 ) and in alkyl halides ( " C—X ) Alcohols and alkyl halides are polar molecules The dipole moments of methanol and chloromethane are very similar to each other and to water... 

The unequal distribution of electron density in covalent bonds produces a bond dipole, the magnitude of which is expressed by the dipole moment, having the units of charge times distance. Bonds with significant bond dipoles are described as being polar. The bond and group dipole moments of some typical substituents are shown in Table 1.7. 

Dipole moment (n ), 38 halomethanes, 335 polar covalent bonds and, 38-39 table of, 39... 

Although the S—O bond lengths in sulphoxides and sulphones seem to indicate that these are covalent double bonds, the dipole moments of these compounds obtained by calculation or by experimental measurements support the semipolar single-bond character in these compounds, and they should be represented as S - O, with about 66% ionic character14,15. 

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